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We have known for a long time that many of the measured white dwarf (WD)masses in cataclysmic variables (CVs) significantly exceed the mean mass ofsingle WDs. This was thought to be related to observational biases, but recenthigh-precision measurements of WD masses in a great number of CVs arechallenging this interpretation. We review the measured WD masses of CVs,determine the WD-mass distribution of an extensive sample ofpost-common-envelope binaries (PCEBs) that are representative for theprogenitors of the current CV population (pre-CVs) and compare bothdistributions. We calculate the CV formation time of the PCEBs in our sample bydetermining the post common-envelope (CE) and the main-sequence evolution ofthe binary systems and define a pre-CV to be a PCEB that evolves into asemi-detached configuration with stable mass transfer within less than the ageof the Galaxy. Possible observational biases affecting the WD-mass distributionfor the pre-CV and the CV samples are discussed. The mean WD mass among CVs is<Mwd>=0.83\pm0.23 Msun, much larger than that found for pre-CVs,<Mwd>=0.67\pm0.21 Msun. Selection effects cannot explain the high WD massesobserved in CVs and we here suggest two possible explanations, both of whichimply substantial revisions to the standard model of CV evolution: either mostCVs have formed above the orbital-period gap (which requires a high WD mass toinitiate stable mass transfer or a previous phase of thermal-timescale masstransfer), or the mass of the WDs in CVs grows through accretion (whichstrongly disagrees with the predictions of classical nova models). Both optionsmay imply that CVs contribute to the single-degenerate progenitors of Type Iasupernovae. The number of He-core WDs found in CVs (<=10%) is roughlyconsistent with the number of He-core WDs in pre-CVs (<=17+-8%), whichindicates a low value of the CE efficiency.

Post-common-envelope binaries from SDSS. XI: The white dwarf mass distributions of CVs and pre-CVs